Processing plated parts



May 29, 1956 M. c. PEASE m, ET AL PROCESSING PLATED PARTS Filed July 20.1951 SURFACE Pl/JTl/VG A/ORM/IZ/ZED 4N0 S/IVTERED l/V MCI/UM INVENTORSJOHN P. JASIONIS ByARSHALL C. PEASE III @fih M; ATTORNEY PROCESSINGPLATED PARTS Marshall C. Pease Ill, Needham, and John P. lasionis,Belmont, Mass, assiguoi's to Sylvania Electric Products Inc., acorporation of Massachusetts Application July 20, 1951 Serial No.237,766

9 (llaim's. (Cl. 204- 37) The present invention relates to theprocessing of metal parts, especially for electric discharge devices andto such devices incorporating parts so processed.

It is frequently required that parts for a vacuum tube or for a tubethat is evacuated and filled with a particular gas at low pressure beplated with another material. This is desirable in the case of tubeshaving an envelope whose wall is of a particular metal which might,except for the plating, be leaky so as to permit gas from outside thetube to enter its interior, or so as to permit the escape of gas thathas been deliberately introduced into the tube.

Metal parts used either for the metal tube wall or for an internalelectrode or other structure are plated for other reasons as well. Inthe course of plating some of the nascent hydrogen that is liberated isquickly absorbed into the bulk of the metal, so as to be a threat to theconditicn of the discharge device in which that part is used. Platedmetals have heretofore been subjected to a sintering operation forforming a continuous seal against the flow of gas through the plate,whether from the exterior of the device into the evacuated space fromthe interior to the exterior, or from the bulk of the metal into thedischarge space.

It has also been the practice of plating parts of metalwalled electricdischarge devices with a metal to make it easier for that part to bejoined to another in a brazing operation. When this is done, there againexists the possibility of nascent hydrogen being absorbed -by the basemetal during the plating operation and forming a pocket between the basemetal and the plated film during the sintering operation. This pocketcould provide a leakage path, in some instances, between the exterior ofthe tube and the interior, in case the pocket appeared in the region ofa braze.

In a particular instance which is discussed in detail below, the polepiece of magnetron (a highly evacuated type of tube used as a microwaveoscillator) is plated and later brazed to other metal parts, the brazingbeing facilitated by the metal plating and the pole piece having beensintered with the intention of insuring the plating against leakage.When such tubes have been so processed there have developed a largenumber of gassy tubes due to leakage past the brazed joints and due tothe presence of hydrogen which, in conjecture, was liberated from thebulk of the pole-piece material or possibly from pockets or blisters inthe plating.

An object of the present invention is to provide greater assuranceagainst penetration of gases through plate on a part exposed to theinternal space of an electric discharge device. More generally, anobject of the present invention is to improve the construction ofelectron discharge devices. A further object is to provide improvedmethods of plating such as to insure uniformity of the bond between theplating and base metal, thereby to eliminate blisters, pockets, and thelike.

The principles of the invention will be described in connection with themanufacture of a magnetron where 2,748,067 Patented May 29, 1956 theinvention has particular merit, but those skilled in the art willrecognize other and more general applica tions of its various featuresand aspects.

In the construction of magnetrons, pole pieces made of cold rolled steelhave heretofore been plated, cover= ing the surface that is later to beexposed to the evac= uated interior of the tube and the surfaces laterto form brazed joints to other metal parts of the envelope wall thatseals off the evacuated interior. Such tubes are of 'Well knownconstruction and are shown, for example, in

Microwave Magnetrons, vol. 6 of the Radiation Laboratory Series,published in 1948 by McGraw-Hill Book Company, and shown at pages 773,774, and 781. A typical tube structure is shown in the accompanyingdrawings, Fig. 1 being a cross-sectional view thereof along the line 11in Fig. 2, and Fig. 2 being a fragmentary longitudinal cross-sectionalong the line 2-2in Fig. 1. These tubes have a pair of pole pieces 10and 12, one of which in the illustrations referred to is formed with abore 14 through which the cathode structure extends, the other alsohaving a bore 16 to make it symmetrical With the first piece and topermit centering of the cathode 18 in the anode structure 20, so thatboth the pole pieces have large surfaces exposed to the internal highlyevacuated space. An electric discharge takes place in this space betweenthe anode and the cathode of the magnetron. The pole pieces havecircular seats 22 that are brazed to cylindrical metal wall struc tures24, that are unitary with the anode itself. This piece is customarily ofoxygen-free copper. In order that the seats of the steel pole pieces maybe effectively bonded as vacuum-tight seals to the copper portion of theevacuated envelope, the pole pieces are usually plated with copper, ormore particularly, with a triple layer of copper, nickel, and copper.

With such construction, it is desirable that the plated surface, both atthe inside area exposed to the evacuated space and the seat Where thebrazing is to be effected, shall be free of blisters and entrapped gas,shall have a firm bond to the base metal, shall be everywhere continuousso as to avoid leaky pores, and shall not entrap any gas or volatilecontaminant that may .be'contained in the base metal.

In applying the present invention, the steel pole piece is plated inconventional manner as with copper, nickel, and then copper again. Thenickel prevents puddling due to excessive flow that might take placewere a simple thick copper plating used alone. This plated part isnormalized in a vacuum of a high order and at a temperature Well belowthe melting or sintering temperature of the plating, a suitable rangefor copper and nickel being 450 to 700 C., at 10- mm. of mercury, forexample. This has the effect of driving off the gases, particularlyhydrogen and other volatile contaminants that might be absorbed in thebulk of the steel or in the plate, or trapped in or under the plate.These may be driven oil? at this stage of the processing since the platehas not yet been sintered and is, therefore, highly porous. The absorbedor trapped gases might, except for this vacuum normalizing operation,later form a blister separating the plating locally from the base metal.After this has proceeded long enough for the vacuum gauge to show thatthere is no further appreciable release of gas, the temperature israised to sintering temperature or to a point just below or even justabove the melting temperature of the plating, 1125-4150 C. for thecopper-nickel-copper plating in this example. The introduction of thenormalizing under vacuum or the pro-sintering bakeout operation has theeffect of greatly improving the plating so as virtually to eliminateblisters, and this in turn refleets on the manufacture of magnetronshaving brazed joints in the envelope wall and plated parts exposed tothe interior, in reducing costly losses due to leakage and accumulationof hydrogen in the discharge space.

In an illustrative procedure for making magnetrons with plated polepieces, pole pieces which have been machined to their final shape arecleaned and degreased in the customary manner, as by successivetreatment in hot carbon tetrachloride, then in a detergent bath,followed by a water rinse, and then an electrolytic cathodic and anodiccleaning sequence. The preparation process also includes firing at 1200C. in a wet hydrogen atmosphere to remove surface carbon and othercontaminants, and as a novel step, in this sequence, firing the parts invacuum at 1200 C.

' The plating of copper on the pole pieces includes further surfacepreparation with hydrochloric acid, a water rinse, and a sodium cyanidedip followed by copper plating. The bath may, for example, be aconventional Rochelle salts plating solution of 57.0 grams per litersodium cyanide, 45.0 grams per liter CuCN, 30.0 grams per liter Rochellesalts, and 20.0 grams per liter NazCOa. The plating is carried out in abath of 60-70 C., at a pH of approximately 12.5 and with a currentdensity of approximately 20 amperes per square foot for two minutes toproduce a plating thickness of about .00005 inch.

Nickel plating is next effected after suitable preparation with waterrinse and acid dips in a bath of 240 grams per liter of NiSO4.6H2O; 45.0grams per liter of NiCl2.6HzO; and 30.0 grams per liter of HsBOs. Thisplating is carried out in a bath of 60-70 C. for approximately 17minutes with an approximate current density of 20 amperes per squarefoot and a pH of 4.5 to yield a thickness of approximately 0.00025 inch.

Subsequently after suitable rinse and cyanide dipping, the copperplating is repeated for a thickness of 0.0002 inch under the sameconditions as above. Thereafter the pole pieces are rinsed, washed, anddried.

This practice is purely illustrative and is entirely unchanged so far asprevious plating practice on magnetron pole pieces is concerned, exceptfor the vacuum firing step in the preparation.

The pole pieces are next subjected to the following novel treatment. Theparts are racked on suitable support in a vacuum of approximately 10*millimeters of mercury, and heated by radio frequency induction. Theheating is controlled at 550-650 C. for a period long enough to insureexpulsion of gases through the plating which is relatively porous atthis stage, usually for 20 minutes. Thereafter the heating is increasedto approximately l125 C.l150 C. to produce sintering or hot flow of theplated metal. This temperature as given in the illustrative example issufficient to produce molten fiow of copper in vacuum, but a lowertemperature for a longer time would produce a comparable sintering evenif the temperature were not enough to allow molten fiow. The criticalrequirement is that sufficient flow be produced by heating so that a gastight skin free of blisters and pores will result upon cooling. Thisheating is continued for approximately minutes, controlling the pressureto prevent an increase above 10- mm. of mercury. The pieces are thenallowed to cool, and the cooling can be expedited through the use ofanhydrous helium or other inert cooling gas.

The fact that the type of normalizing and sintering process describedyields parts of greatly improved plating characteristics, especially byobtaining good adherence of the plated metal to the base metal and bythe virtual elimination of blisters and by the effective elimination ofabsorbed gas is of great importance to the art of tube fabrication.

The pole pieces of the magnetron used as an illustrative but importantexample have an area machined to form a circular seat which is designedto mate with and be brazed to a corresponding area of a cylindricalcopper body part which, together with other parts of the tube form thevacuum envelope. This joint is commonly formed by brazing the partstogether with a copper-silver eutectic alloy in a hydrogen furnace. Sucha braze is difficult to make directly to the steel. It is much moreeasily and reliably made if the steel is plated by a process whereby asurface layer of sintered copper-nickel-copper alloy is formed. It isessential, however, that said layer of alloy be well adherent to thebase metal so that no leak may occur there. Any leak in the joint, eventhough it be under the sintered plated alloy, will cause the destructionof the tube.

Blisters on the surface of any area internal to the vacuum envelope willalso be sufficient, in general, to destroy the tube. Such blistersnormally are filled with gas, thought to be usually hydrogen. During theprocess of exhausting the tube it will normally be baked at a somewhatelevated temperature for some time at the same time as it is beingpumped to a high vacuum. Two hours at 450 C. at a pressure below 10' mm.of mercury may be taken as illustration. In as much as the gas enclosedin a blister is enclosed by a dense and relatively impervious alloy ofthe sintered plate, said gas will not, in general, be removed by saidprocess of baking out. After the tube is finished, however, and issealed off, it is expected not to lose the degree of vacuum required forsatisfactory operation which, by Way of illustration may be of the orderof 10* mm. of mercury, even though the tube be kept for several monthsor even years. Over such a long period of time the gas contained in ablister will diffuse through the metal into the supposedly evacuatedregion of the tube causing the tube to fail to operate satisfactorily.

In addition to the slow process of diffusion described above, the gascontained in a blister on an interior surface may be released into thesupposedly evacuated region and prevent satisfactory operation by therupture of the blister at any time. This may be the result, for example,of the occurrence of a momentary are at the blister.

Finally, even if said plating adheres well to the base metal and noblisters have been formed anywhere on the internal surface there isstill the possibility of gas being entrapped under the sintered platingby direct solution in the base metal. This gas may originate fromimpurities present in the base metal, from the nascent hydrogen presentduring the plating operation, or from the hydrogen atmosphere usedduring the brazing and other operations of assembly. This hydrogen orother gas may be present under the sintered layer of alloy and may notbe removed by the usual exhaust procedures. It is thought, in fact, thatthe hydrogen is the cause of the blistering observed in the methods ofprocessing prior to this invention. It is thought that these blistersare formed as the result of the pressure generated by hydrogen absorbedin the base metal. But even if no blisters occur the hydrogen or othergas may remain entrapped under the sintered plated alloy and by slowdiffusion into the evacuated space, ultimately prevent the tube fromoperating satisfactorily.

Control of gas in a vacuum tube is, therefore very important. If excessgas develops in the evacuated region after the tube has been sealed off,the tube is no longer satisfactory. It can not usually be salvaged andmay fail under hazardous circumstances. Any method of processing theparts, therefore, that helps to reduce the incidence of gas in finishedtubes by eliminating one or more possible source of gas is valuable.

The present invention is not necessarily identified with the metals andconditions described, although the plating metal or metals should have alower sintering temperature than the melting point of the base metal.The process is used to best advantage to make parts from metals whichhave a pronounced tendency of absorbing hydrogen, such as iron, nickel,cobalt and various alloys thereof. The plating metals including platedmetal alloys should be such as form a relatively hydrogen tight surfaceseal, including as examples the copper-nickel plating described, gold,gold alloys including gold-indium. To those skilled in the art, somemetals are known to pass hydrogen freely at elevated temperatures(palladium for example) and such metals would naturally not be chosen asplatings to form the desired surface seal.

While described in connection with a high-vacuum tube electric dischargedevices such as transmit-receiver tubes containing hydrogen or other gasat low pressure can also be made to advantage with the novel featuresdescribed.

Various other applications may be made of the foregoing novel conceptsand a variety of modifications and substitutions will occur to thoseskilled in the art; and therefore the appended claims should be alloweda broad latitude of interpretation, consistent with the spirit and scopeof the invention.

What we claim is:

1. The method of processing a plated metal part to be exposed to theinterior of an electric discharge device, including the steps ofelectroplating the part of a base metal with a porous layer of adifferent metal whose melting temperature is lower than that of saidbase metal, heating the plated part in vacuum over a period and at atemperature sufiicient to remove the gasses contained in the partWithout however sintering the plating, and thereafter raising thetemperature suflicient to sinter the plating while maintaining a vacuumof a high order.

2. The method of processing plated metal whose plating has a lowersintering temperature than the melting temperature of the base metal,which includes the steps of treating the plated metal in vacuum at ahigh temperature inadequate to produce sintering for a sufficientlyextended period to reduce the rate of removal of occluded gas to anegligible level, and thereafter raising the temperature at least to thesintering temperature of the plating but below the melting temperatureof the base metal while maintaining a vacuum.

3. The method of assembling a vacuum tube, the envelope of whichincludes multiple metal parts joined together by brazing, one of whichparts is of steel, which 6 method includes the steps of plating thesteel part to be brazed, heating the plated steel part to 450-700 C. ina high vacuum for a pericrl suflicient to draw out the free gasses,without sintering the plating, thereafter sintering the plating atapproximately 1125 C., and cooling the part in an inert atmosphere.

4. The method in accordance with claim 1 in which said part iselectroplated with copper.

5. The method in accordance with claim 1 in which said part iselectroplated with nickel.

6. The method in accordance with claim 1 in which said part is of steeland is electroplated with copper and nickel.

7. The method in accordance with claim 4 wherein the removal of gases iseffected at a temperature in the range 450 C. to 700 C. and in which thesintering is eifected at a temperature in the range 1125 C. to 1150 C.

8. The method in accordance with claim 6 wherein the removal of gases iseffected at a temperature in the range 450 C. to 700 C. and in which thesintering is effected at a temperature in the range 1125 C. to 1150 C.

9. The method in accordance with claim 3 in which copper is a metalelectroplated on the steel part.

References Cited in the file of this patent UNITED STATES PATENTS2,077,633 McMaster et a1. Apr. 20, 1937 2,128,234 Dallenbach Aug. 30,1938 2,424,576 Mason July 29, 1947 2,478,534 Kather Aug. 9, 19492,490,700 Nachtman Dec. 6, 1949 2,512,141 Ma et a1. June 20, 19502,535,713 Wooten Dec. 26, 1950 2,556,864 Apker June 12, 1951

1. THE METHOD OF PROCESSING A PLATED METAL PART TO BE EXPOSED TO THEINTERIOR OF AN ELECTRIC DISCHARGE DEVICE, INCLUDING THE STEPS OFELECTROPLATING THE PART OF A BASE METAL WITH A POROUS LAYER OF DIFFERENTMETAL WHOSE MELTING TEMPERATURE IS LOWER THAN THAT OF SAID BASE METAL,HEATING THE PLATED PART IN VACUUM OVER A PERIOD AND AT A TEMPERATURESUFFICIENT TO REMOVE THE GASSES CONTAINED IN THE PART WITHOUT HOWEVERSINTERING THE PLATING, AND THEREAFTER RAISING THE TEMPERATURE SUFFICIENTTO SINTER THE PLATING WHILE MAINTAINING A VACUUM OF A HIGH ORDER.